Kamuysaurus japonicus

Some of you may have come across online photos or information concerning a dinosaur skeleton from Japan informally named "Mukawaryu" ("Mukawa dragon", "-ryu" for "dragon" being the traditional suffix for informally named Japanese dinosaurs). The specimen went from "informally named" to getting a real scientific name in just a couple of years, exchanging "Mukawaryu" for Kamuysaurus japonicus.

"Mukawaryu", now Kamuysaurus japonicus. Figure 3 in Kobayashi et al. (2019) (go to link for full caption). The scale bars are all 10 cm (4 in) except for the skeleton itself (b), which is 1 m (40 in). CC-BY-4.0.

Genus and species: Kamuysaurus japonicus. The genus name is a reference to the kamuy, spiritual entities of the Ainu people, indigenous to Hokkaido. The species name refers to Japan (Kobayashi et al. 2019). Together, we get something like "Kamuy lizard from Japan".
Citation: Kobayashi, Y., T. Nishimura, R. Takasaki, K. Chiba, A. R. Fiorillo, K. Tanaka, T. Chinzorig, T. Sato, and K. Sakurai. 2019. A new hadrosaurine (Dinosauria: Hadrosauridae) from the marine deposits of the Late Cretaceous Hakobuchi Formation, Yezo Group, Japan. Scientific Reports 9, article number 12389. doi:10.1038/s41598-019-48607-1.
Stratigraphy and geography: The type and only known specimen comes from an outcrop in the middle of the IVb unit of the Hakobuchi Formation (Yezo Group), regarded as earliest Maastrichtian in age. The surrounding rock was a sandy marine mudstone bed. The IVb unit has also yielded ammonites, a sea turtle, and mosasaurs, and is thought to have been deposited in an outer shelf setting. The discovery site was "along the Shirafunezawa Creek in the northern Hobetsu area of Mukawa town in Hokkaido" (Kobayashi et al. 2019).
Holotype: HMG-1219 (Hobetsu Museum), a nearly complete, semi-articulated skeleton. Missing elements include the muzzle of the skull, the sacrum, the end of the tail, and some of the chevrons. Some of the bones were affected by bio-erosion before burial (Kobayashi et al. 2019).

Not only is K. japonicus known from a nearly complete skeleton, but the skeleton is the most complete reported to date from Japan for any classic dinosaur, bypassing Fukuivenator paradoxus. (And I totally called Fukuivenator a couple of years before it was published.) As a Maastrichtian dinosaur, it is also the most geologically recent from Japan. Getting it out of the Yezo Group is a bit of a fluke; the Yezo is a marine unit, and the type specimen of K. japonicus appears to have been a bloat-and-float, as depicted in Figure 7 of Kobayashi et al. (2019).

The nasals and the premaxillae are missing, which is a shame because a lot of interesting things go on with those bones in hadrosaurs. Kobayashi et al. (2019) interpreted the structure of the neighboring bones as indicating the presence of a short, sub-Brachylophosaurus crest over the "forehead". Otherwise, the skull is broadly your basic saurolophine/hadrosaurine skull, albeit perhaps a bit short and deep in its proportions. The rest of the skeleton is also fairly typical for a saurolophine. The neural spines on the dorsals are on the long side, but not in a notably exaggerated way. The right femur is 1095.0 mm long (43.11 in) per the supplementary information (never forget the supplementary information!), which Lull and Wright (1942) tells me would be right in line for most grown North American hadrosaurs. The right tibia is 1035.0 mm long (40.75 in), making the tibia/femur proportion closer to 1 than most saurolophines, but like the neural spines not unusually so (~95% vs 80%–90% in most saurolophines, interestingly more like a lambeosaurine; I had no idea that there was a difference). Histologically, Kobayashi et al. (2019) found that the holotype was essentially fully grown. They estimated its size as around 8 m long (26 ft) with a body mass of 4.1–5.3 metric tons (4.5–5.8 US tons), depending on the habitual stance (biped vs quadruped).

The skull of K.japonicus, figure 2 in Kobayashi et al. (2019) (go to link for full caption). Scale bars are all 5 cm (2 in) except for i and j, which are both 1 cm (0.4 in). CC-BY-4.0.

Although Kobayashi et al. (2019) suggested that K. japonicus had a small cranial crest perhaps like that of Brachylophosaurus, their phylogenetic analysis found it to group with the edmontosaurs, famous for having no bony crests at all. K. japonicus also is unusual in retaining or reversing to some characteristics found farther back in the iguanodont tree, such as the slenderness of its humerus and the sub-rectangular shape of its infratemporal fenestra. To clarify one other point, Kobayashi et al. (2019) referred to K. japonicus and companions as hadrosaurines rather than saurolophines. This is because in their analysis Hadrosaurus has opted to commit to its old home rather than do its own thing outside of the lambeosaurine/saurolophine split. Its penchant for jumping positions, and the resulting changes in terminology, is one of the annoyances of ornithischian classification (though not nearly as bad as the ongoing "titanosaurization" of the old hypsilophodonts).

References

Kobayashi, Y., T. Nishimura, R. Takasaki, K. Chiba, A. R. Fiorillo, K. Tanaka, T. Chinzorig, T. Sato, and K. Sakurai. 2019. A new hadrosaurine (Dinosauria: Hadrosauridae) from the marine deposits of the Late Cretaceous Hakobuchi Formation, Yezo Group, Japan. Scientific Reports 9, article number 12389. doi:10.1038/s41598-019-48607-1.

Lull, R. S., and N. E. Wright. 1942. Hadrosaurian Dinosaurs of North America. Geological Society of America Special Paper 40.